About thin provisioning

You can use thin provisioning technology to more efficiently allocate storage space, while still meeting application and user storage needs. With a thin-provisioned volume, the group allocates space based on volume usage, enabling you to “over-allocate” or “over-provision” group storage space.

Recommendation: Dell recommends that you fully understand the benefits and risks of using thin provisioning before implementing it in your environment. Environments that use thin provisioning should have around-the-clock support to handle any space allocation issues and prevent service level disruption.

Thin provisioning volumes is beneficial in a number of environments. For example, if your environment does not easily allow you to expand file systems or raw disks, you can give thin-provisioned volumes excessively large reported sizes to account for future growth. The group automatically allocates space to volumes only if usage patterns warrant the space.

Thin provisioning also helps you plan for future group expansion. For example, you can size volumes according to their maximum possible space requirements, even if the group currently cannot provide all the required space (that is, you can “over-provision” group space). As volume usage increases, you can expand group capacity, with no user impact. You do not need to change drive letters, expand volume sizes, or add volumes.

Thin provisioning is most effective if you can accurately predict how volume usage increases over time.

When you create a volume, you specify the reported size for the volume. The reported size is seen by iSCSI initiators. The actual amount of pool space that the group allocates to a volume is called the volume reserve. The value of the volume reserve depends on whether you enable thin provisioning on a volume:

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No thin provisioning – The volume reserve is equal to the reported size.

For example, even if only 10% of a volume is in use, the group allocates the full reported size.

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Thin provisioning – The volume reserve is equal to or less than the reported size, depending on volume usage and the thin provisioning settings.

Initially, the group allocates the minimum amount of volume reserve for a thin-provisioned volume. The minimum is 10% of the reported volume size or the user-specified percentage.

As initiators write to the volume, free volume reserve decreases. When free volume reserve falls below a threshold, the group increases volume reserve, up to a user-defined maximum (assuming available free pool space):

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For a volume with a reported size of 100GB or greater, when free volume reserve is less than 6 GB, the group allocates an additional 10 GB.

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For a volume with a reported size that is less than 100GB, when free volume reserve falls below 6% of the reported volume size, the group allocates an additional 10% of the reported volume size.

Event messages inform you when in-use volume reserve surpasses a user-defined limit and reaches the maximum.

Thin provisioning space settings

Three settings control how the group allocates space to thin-provisioned volumes and when the group generates events related to space usage:

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Minimum volume reserve – Minimum amount of pool space that the group allocates to the volume, based on a percentage of the reported volume size. The default group-wide setting is 10%.

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In-use space warning limit – Amount of in-use volume reserve that results in notification, based on a percentage of the reported volume size. The default group-wide setting is 60%.

For volumes larger than 200 GB, when the in-use volume reserve increases by every additional 10 GB.

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For volumes smaller than 200 GB, when the in-use volume reserve increases by every additional 5%.

For example, if you create a thin-provisioned volume with a size of 500 GB and set the warning limit to 75%, a warning occurs when the amount of in-use volume reserve is more than or equal to 75% of 500 GB, or 375 GB.

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Maximum in-use space – Maximum amount of in-use volume reserve (maximum size of the volume reserve), based on a percentage of the reported volume size. The default group-wide setting is 100%.

The maximum in-use space value determines the behavior when the volume reserve reaches its maximum size:

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If the maximum in-use space value is less than 100%, and an initiator write exceeds this limit, the write fails. The group sets the volume offline and generates event messages.

If you increase the maximum in-use space value or the reported volume size (both operations require free pool space), the group automatically sets the volume online and writes succeed.

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If the maximum in-use space value is 100%, and an initiator write exceeds this limit, the volume is not set offline; However, the write fails, and the group generates event messages. If you increase the reported size of the volume, writes succeed.

This behavior is the same as when in-use space for a volume that is not thin-provisioned reaches its reported size.

The maximum in-use space value helps prevent one volume from consuming all the pool free space and setting other thin-provisioned volumes offline.

You can change the group-wide default volume settings (see Modifying group-wide volume settings), override the default values when you create a thin-provisioned volume, or modify a volume and change the settings.

Enabling thin provisioning on a volume

When you create a new volume or clone an existing volume, you can enable thin provisioning on the volume. In addition, you can modify an existing volume and enable thin provisioning.

Thin provisioning is not appropriate for all environments or volumes. You must fully understand thin provisioning before implementing the functionality on a volume. See About thin provisioning.

When enabling thin provisioning on an existing volume, be aware of these issues:

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Enabling thin provisioning on a volume usually decreases the amount of space that the group allocates to the volume (called the volume reserve).

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Enabling thin provisioning changes the amount of allocated snapshot space and replication space, because the group allocates snapshot space and replication space based on a percentage of the volume reserve.

However, the group increases the snapshot space and replication space percentages to prevent the deletion of snapshot or replication data.

To enable thin provisioning on an existing volume:

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Click Volumes in the lower-left panel, then expand Volumes, then select the volume, then click Modify settings, and then click the Space tab.

If you disable thin provisioning on a volume, the group allocates the full reported volume size (the reported size and the volume reserve is the same). Therefore, you must have sufficient free pool space.

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Because the group bases snapshot space and replication space on a percentage of the volume reserve, disabling thin provisioning increases snapshot space and replication space. Therefore, you must have sufficient free pool space.

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In some cases, if you disable thin provisioning on a volume, the group automatically decreases the snapshot reserve percentage to prevent an excessive allocation of snapshot space.

This can occur if you previously set the snapshot reserve percentage to a high value to prevent the group from deleting snapshots (for example, if you increased the snapshot reserve percentage from 100% to 500%). If you disable thin provisioning on the volume, the group might decrease the percentage to a more appropriate value, closer to 100%. The group does not decrease the snapshot reserve percentage if the decrease requires deleting snapshots.

To disable thin provisioning on a volume:

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Click Volumes, then expand Volumes, then select the volume, then click Modify settings, and then click the Space tab.